The invention relates to a reflecting telescope with a primary mirror formed of individual, adjustably mounted mirror segments, with a tube positioned concentrically with respect to a primary mirror and equipped with observation cages, and with a system of mounting bars for the tube.
A reflecting telescope of this generic kind is exemplified by the Mauna Kea reflecting telescope with a reflecting diameter of 10 meters, which is still in the planning stage and is intended to be the largest reflecting telescope in the world to date (Sterne und Weltraum [Stars and Space], 1984, 8 to 9, p. 412). In that reflecting telescope, the primary mirror is formed of 36 hexagonal mirror segments that form the surface of the mirror in a honeycomb arrangement, with one mirror segment left open in the middle for observation in Cassegrain focus. The fabrication of the individual hexagonal mirror segments alone is highly problematic. The latter are off-axis sections of a paraboloid that must be cut with six corners. Production starts with a circular blank that is deformed by precisely defined shearing and bending forces applied to the edge. A spherical shape is ground into the deformed blank. The forces applied are then again removed. If the forces have been accurately chosen, the mirror segment when released takes on the desired shape of a paraboloid section. It has been found, however, that during cutting into the hexagonal shape warping may occur, so that the production of the different mirror segments is very costly. In addition, the position of the individual hexagonal mirror segments must be adjusted subsequently depending on the position of the telescope, wind load and temperature fluctuations. Hence, the support points of each mirror segment are connected with three position regulators that refocus the mirror segment and can be moved in two inclination directions. On the edges on the reverse side are sensors that measure the shifting of adjacent mirror plates with respect to one another. In conjunction with three inclination sensors that measure the total curvature of the mirror segment, they supply data that is processed in a computer system that controls all 108 position regulators. With a total of 168 different sensors, the redundancy is so great that the failure of individual sensors can be coped with. At any rate, with this arrangement the fronts of the mirror segments are free of interfering control systems, but it is necessary on occasion to readjust it with the aid of a star configuration so that infra-red observations during the day are also possible. Sensors and position regulators must work accurately at least 50%.
In addition, one-piece primary mirrors for reflecting telescopes are known, such as the 3.5 meter telescope at the Max Planck Institute of Astronomy (Zeiss Information No. 94, 1982, pp. 4 ff). In that telescope, the mirror element is made of a glass ceramic Zerodur) with a low temperature expansion coefficient. The surface of the mirror is ground as a high order hyperboloid of revolution and polished. The standard deviation is 30 nm at most; measurement is done by laser interferometer. The mirror element is mounted on an 18-point bearing.
Such reflecting telescopes must meet the following requirements:
1. As much radiant energy as possible must be collected; this is proportional to the light-gathering surface of the primary mirror. It follows that the diameter of the primary mirror must be as large as possible. Diameter is limited, however, by technical and economic conditions.
2. The radiation picked up from a star should be concentrated as sharply as possible at a point within the focal plane of the reflecting telescope. The quality of the optical image should be as good as possible. Problems result with earth-bound reflecting telescopes from the influence of the earth's atmosphere, e.g., air turbulence. Reflecting telescopes on satellite orbits, however, do not experience these problems.
3. The star to be viewed should be held as long as possible within the image plane without position changes.
The disadvantage of using one-piece primary mirrors in reflecting telescopes is that because of reasons related to their production they are limited in diameter. The largest reflecting telescope with a one-piece primary mirror is the Hale telescope, which has a mirror diameter of 5 meters. However, the only reflecting telescope known to date with a primary mirror made up of individual mirror segments has the disadvantage that the production of the individual mirror segments is extremeLy costly, since each mirror segment must be ground by itself as a section of a paraboloid. Furthermore, the sensors on the lines where the individual segments touch must insure a precise adjustment. The expense thus involved in supplementary technical arrangements is extremely high in relation to the overall effectiveness of the telescope. In addition, a highly complex mounting system is required for the segments.
Both kinds of reflecting telescope suffer from the disadvantage that the observation cages situated inside the tube cast a shadow with their entire diameter over the reflecting surface of the primary mirror. At the same time, the mounting bars cast other shadows that extend to some extent radially over the primary mirror. These shadows occur and are unavoidable with all types of focusing. They bring it about that reflecting surfaces of the primary mirror that were produced at great cost are ineffective, because they cannot contribute to the collection of the radiation energy.